An optical time-domain reflectometer (OTDR) is a useful tool for testing point-to-point fiber optic links, testing passive optical networks (PONs), and finding faults, such as breaks and measure reflectance or optical return loss (ORL) in fiber optic networks. As shown in FIG. 1 a typical OTDR includes one or more processors, which include necessary CPU, memory, software and other components. These processors are in communication with a user interface made up of a display and controls, such as buttons, knobs, touch screens or the like, an output pulse generator and laser driver, a sampling analog to digital converter and a photo detector with input amplifier. The output pulse generator and laser driver drives the laser or lasers, which send light pulses through a coupler and into an optical fiber network under test.
The OTDR generates output pulses and measures the return signal from the same end of the fiber network under test. The return optical signal from each pulse (optical fiber backscatter and event reflections) is converted into an analog electrical signal that must be amplified. The OTDR input amplifier has a finite gain-bandwidth product, which is a key reason why real OTDR event resolution is longer than would be indicated by the nominal pulse width in nanoseconds divided by 10 equals trace event width in meters. The amplified electrical signal is sampled and converted into digital data points, which are averaged over many pulses (determined by test duration) and digitally filtered if needed to decrease noise, and thus increase dynamic range, at the expense of resolution. The product is a trace.
OTDRs typically include a number of user settings, including setup mode, wavelength, distance range, pulse width, and test duration. The setup mode setting allows the user to configure the OTDR for automatic mode, manual or expert mode, or, in some models, application-specific operations. The wavelength setting determines which lasers (wavelengths) are used to measure the fiber network under test. The distance range setting determines the section (length) of the fiber network under test that will be included in the OTDR trace. The pulse width setting determines test pulse duration, typically set in nanoseconds or microseconds, and typically also determines other key OTDR parameters such as amplifier bandwidth and digital or analog filter settings. The test duration setting determines the nominal duration of the test, typically set in seconds or minutes, which in turn determines the number of samples averaged to create each trace data point.
In the automatic mode, most settings are selected automatically by the OTDR. This is useful for inexperienced users, or experienced users who need to create a baseline trace quickly, but will result in an OTDR setup that is not optimized for most applications. FIG. 2A shows typical OTDR user settings in automatic mode. In the expert mode, most or all settings are selected by the user. FIG. 2B shows typical user settings in expert mode. Additional setting choices offered in the expert mode will allow experienced users to optimize the performance of their OTDR for specific applications, such as detecting and/or resolving closely spaced events or measuring the loss of a splice near the far end of the fiber network under test.
OTDR dynamic range is a key performance parameter. OTDR dynamic range is a function of pulse width, test duration, and OTDR signal processing parameters typically automatically selected by the OTDR based on pulse width, such as input amplifier and filter (analog, digital or both) bandwidth. Therefore, in the expert mode of a current art OTDR, the user determines dynamic range by selecting pulse width and test duration. For a given choice of distance range and wavelength, each pulse width/test duration combination corresponds with a given OTDR dynamic range. The higher the dynamic range, the smoother the OTDR trace, and therefore the better the loss measurement accuracy, which is needed to detect and measure low loss events, such as splices. The user can increase dynamic range by increasing pulse width, which coarsens event resolution, or increasing test duration, or both. Therefore, OTDR setup involves a tradeoff between dynamic range, event resolution, and test duration, but only test duration is a parameter available for viewing and adjustment on current art OTDRs.
The tradeoff between dynamic range and event resolution is explained with reference to FIGS. 3A-3C. FIG. 3A shows an exemplary fiber network 70 being tested by an OTDR 72. The network includes an input connection 73, two connections 74, 75, a splice 76 and a network termination 77. FIG. 3B shows a trace 80 produced by the OTDR when a fine event resolution is chosen by the user. This trace 80 allows the user to clearly differentiate between the two connections, shown in FIG. 3B as events 81, 82, but user cannot accurately measure the loss of the splice, which is a low loss event. FIG. 3C shows a trace 90 produced by the OTDR when a high dynamic range setup is chosen by the user. This trace 90 allows the user to accurately measure the loss of the splice, which is the downward sloping region 91 on the trace. However, the two connections are shown on the trace as a single event 92 rather than two separate events.
Pulse width is not the same as event resolution. The idealized relationship between OTDR trace event width in units of distance (d) and output pulse width in units of time (t) is d=ct/2n, where c=the speed of light in a vacuum and n is the fiber group index of refraction at the trace wavelength. This relationship is often approximated as d (m)=t (ns)/10, as discussed above. Common measures of event resolution, such as attenuation dead zone, are normally longer than the idealized trace event width because they include the effects of output pulse shape, input amplifier bandwidth, filtering (analog, digital, or both) and other OTDR characteristics. Therefore pulse width is not the most precise metric for representing event resolution.
Therefore there is a need for an OTDR that provides for the display and adjustment of dynamic range and a measure of event resolution in its users settings, that shows dynamic range in setup menus and ties this value to the current pulse width and test duration settings, that allow the user to change dynamic range directly and tie this new setting to event resolution and test duration, and that also allow the user to ‘lock’ any one of these settings, the one most important to the user in the given application, and then make trade-offs between the other two.